Busy-test for telephone systems.



E. R. CORWIN. BUSY TEST FOR TELEPHONE EYSTEMS.

APPLICATION TILED mm. 20, won. ammwm NOV. 2, 1:110.

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ELMER R. COR'WIN, OF CHICAGO, lTl'iLIllDIf-J', .AivEIUNOP "lt') L'JOl't'WIlIN TELEPHONE MANU- FACTURING COMPANY, OF C'E-IICAGO, IL'

BUSY-TEST FOITL TELIEiIP't-IUNIE: S'f'E'lZt-lhttfri.

Specification of Letters Patent.

Patentedtint.8,1912.

Application filed February 20, 1908, Serial No. 416,857. Renewed November W10. Serial No. 590,384.

To all whom it may concern.

Be itlmown that I, ELMER R. Ionwrn, a citizen of the United States of inner-lea, re siding at Chicago, in Cook county, Illinois, have invented a new and useful Busy-'lcst for Telephone Systems, of which the follow' ing is a specification, reference being had to the accompanying drawing, illustrating same.

My invention relates to telephone systems and deals principally with the busy-test for such systems.

The principal objects of my invention are to provide an improved busy-test arrangement for multiple telephone systems; and to provide improved means or automatically controlling the busy-test arrangement in such systems. i

Other objects will be apparent from the following specification.

In the telephone system illustrated in the accompanying drawing I supply a ringing relay and an operators disconnecting relay, both associated with a plug of the 'ilug-cord connecting circuit, with current from a line connection extending through a third conductor of the cord, plug and any one of a plurality of line jacks, for operating the said relays when connection is made with the line. Then I provide a locking relay in the plug-cord circuit and supply this relay with current through the said line connect-ion when the subscriber removes his to cciver from the switch-hook while connection is made with the line, to cause the latter said relay to lock the ringing relay in a non-ringing position until the connecting plug is removed from the line jack. The operators disconnecting relay in addition to controlling the operators circuit also helps control a supervisory signal. I also provide a supervisory relay associated with the said plug of the plug-cord circuit, for

partially controlling the supervisory signal, and a line relay, through both of which relays the subscribers line is supplied with current for talking purposes during coir versation. The stuocrvisory relay in addition to cont-rollingthe supervisory signal also places the locking relay in locked positicn when the subscrilmr removes his re-- ceivcr from the switclnhoolc while connection is made with the line.

I preferably provide each of the pair of plugs in a plug-cord connecting circuit lwith all o'l' the relays above mentioned, to provide automatic ringing For each of the plugs and to properly control the operators circult and suptu'visory signals as will be hereinafter described, but l wish it to be understood that 1 may do away with the ringing relay and the locking relay associated with one oi. the plugs, if it is desired to ring out over only one plug of the pair.

In the operators telephone set I provide an impedance coil connected from one pole of a central source of elcctiricitr to a medial point of the operations telephone rct'ciycr winding, the latter being normally con-- ncclcd with the tip strands ol the connecting plugs, so as to provide a path for the busyd'est currents which extends through the said. impedance coil, a portion of the operatofis receiver winding and a lip strand of either cord and plug of a pair. The busy-test is facilitated by the change of potential of the line-jack test terminals, which indicates the condition oi. the line, in the operators receiver, when the tip of a connecting plug is connected with a linejach test terminal of the line. The t'merators disconnecting relays control the busy'tcst circuit over the respective connecting cords and plugs of a pair.

I will now describe my invention more in detail, by reference to the accon'ipanying drawing which illustrates two subscribers telephone lines and the plug-cord connecting circuit therefor, the whole being adapted. and provided with apparatus, to carry out my present invention. The particular line circuit and connecting circuit herein illustrated forms the subject matter oi my copending application for Letters Patent on telephone system tiled herewith.

in the drawings the lines A and B are each provided with a substation at which the circuit of the line is closed, when the receiver is removed from the switclnhook, in the usual manner, the circuit of the line being open when the receiver is hung upon the switclrhook. Limb 1 ol. each line ere tends to terminals 3 $3 of line spring-jacks C G, in multiple, and also through a line relay ($0 to pole hot a central. battery F. Limb 3 of each line extends through a normally closed contact tll flil of? a cutoll we lay (B -l topole 'ltl'o't' battery l liclay 63 is connected from pole 10 ol battery i to terminals "i 1 of the jacks C C, in multiple.

When relay 60 is energized it closes contact 6465 in the circuit of a line lamp 7. When relay 63 is energized it opens contacts 61-62 and 61-66, and closes contact 6167, contact 61-6 6 being in the circuit of line lamp .7 and contact 6167 being in circuit with the terminals 5 of the jacks C C, which terminals 5 5 are in multiple.

In the plug-cord connecting circuit the talking strands 11 and 12 of each plug D and E are connected together through condensers 13 and 14, respectively. Each plug I D and E has a ringing relay 15 associated therewith to control. suitable contacts; When either of the relays15 15 is energized, spring members 16 and 18 thereof are caused to engage contacts 20 and21, respectively, which are connected through conductors 22 and 23, respectively, to a ringing generator G, spring member 16 also disengaging contact 17. Each of the plugs D and E is provided with a third strand or conductor 24 adapted to furnish a con nection through any one of the several j acksprings 4 4 when the plug is connected with a line jack, to furnish current supply for operating the several relays in the plugcord circuit. A pair of operators'discom nect relays 25 and 26 are connected from the respective strands 24 24 of the plugs D and E, to pole 9 of battery F, such connections causing these relays to become ener gized when their respective plugs are connected with line jacks. Thesecondary 68 69 of an operators induction coil, an op erators receiver 70, and a condenser 71, are bridged from talking strand 11, on opposite sides of condenser 13, to talking strand 12, on opposite sides of condenser 14, preferably as shown, through normally-closed contacts 30--31 and 3233 of each of the relays and 26, in multiple, so that the operators set will be entirely disconnected from the talking strands 11 and 12 of the cord circuit when both of the relays 25 and 26 are energized. An impedance coil 75 is connected from between the magnet coils of the receiver 70, through conductor 39 to pole 9 of battery F. This coil 75 is to enable a busy-testto be made as hereinafter described.

The arrangement of the operators relays 25 and 26, is practically the same as in my copending application for Letters Patent on telephone system, Serial No. 414,078, but in addition to controlling the operators circuit these relays are in this present application also arranged to control the supervisory signals as will. be hereinafter described. W hen either of the relays 25 or 26 is energized it closes a contact 72-73 in the corresponding supervisory signal circuit. To

each strand 24 of the plugs D and E is con nected the corresponding ringing relay 15, through a normally-closed contact 3435 of a locking relay 36, the circuit from each ringing relay 15 extending through conductor 37 rotary intermittent circuit-closer H, and conductor 39, to pole 9 of battery F. Each supervisory lamp 40 is connected through conductor 38 to pole'lO of battery F. From talking strand 12, on opposite sides of conductor 14, are connected a pair of supervisory relays 43 43, to pole 10 of battery F. When either of the supervisory relays 43 43 is energized it causes spring member 42 thereof to disengage contact 41 and to engage contact 46 which leads to the winding of the corresponding locking relay 36. Thus in addition to controlling the supervisory signals, the relays 43 43 are also arranged to control the. circuits of the respective locking relays 36 36. When either of the locking relays 36 36 is energized it causes spring member 34 thereof to disengage contact 35, and spring member 74 to engage contact 47, spring member 34 being connected to the corresponding strand 24 and contact 47 being connected to pole 9 of battery F.

The operation of the system is as follows: When a party at substation A desires to make a call, he lifts his telephone receiver from the switch-hook in the usual manner,

which closes the circuit between limbs 1 and 2 of his line and thereby closes a circuit from pole 9 of battery F, through his line relay 60, line A, and contact 6 162, to pole 10 of battery F. Current from battery F flowing through this circuit energizes line relay 60 and thereby causes same to close contact 64'65 which bridges the line lamp 7 through contacts 66-61--62 across battery F. Current from: battery F now flowing through line lamp 7 lights thislamp which signals the operator at the central station for a connection, whereupon the operator inserts one of the plugs D or E into a jack C of line A to answer the call." The plu contacts 11,- 12 and 24 now engage jac terminals 3, 5, and 4, respectively, thereby closing a number of circuits through battery F, which circuits will now be traced and explained as follows:

First. From pole 9 of battery F, through conductor 39, one of the operators disconnect relays 25-01 26 (say 25), strand 24 of the connected plug (say D), a jackspring 4, and cut-ofl relay 63, to pole 10 of battery F. Current from battery F flowing through this circuit energizes relay 25 and thereby causes its armature to be attracted and to open contacts 30 31 and 3233 and close contact 72-73. Although contacts 30-31 and 3233 of relay 25 are now open, which disconnect, the operators telephone set and the busy-test arrangement from plug 1), the operators telephone set is still bridged across the plug-cord talking circuit from talking strand 11, through contacts 30.3l and 32-33 of relay 26, to talking strand 12. Contact 7 2-73 being closed does not cause the supervisory lamp &0 associated therewith to light, because contact ll--42 is simultaneously opened as Wlllbfi licreinbelow described. The current now flowing in circuit --39, 25, 24', 4t, 63, also energizes the cut-oil relay 63 and thereby causes its armature .xlfiecond. From pole 10 f b tt y Ii through supervisory relay 43 on the con- "nected-plug-side oi. the (gordwircuit, strand 12, a jack terminalb contact (KP-61, line A, and line relay 60, to pole 9 of'battery F. Current from battery F flowing through this circuit energizes relay 43 and thereby causes its armature to beattracted and to open contact e1- e2 and close contact Contact 41-4l2 now being open prevents the supervisory lamp 40 associated therewitlrfrom displayinga signal. Contact 42-t6 being closed closes a circuit from pole 9 of battery F, through contact 42-4l6, locking relay 36,strand 2 1 of the connected plug, a jack terminal fl, and cut-oil relay 63, to pole 10 of battery F. Current from battery F flowing through this latter circuit energizes relay 36'z1nd thereby causes its armatures to be attractedand to opencontact 3 1 435 and close contact 74-47, respectively. Contact 34-435 now being open prevents ringing relay 15 from becoming energized. Contact 74*47 being closed closes a circu'it from pole 9 of battery F, through contact LT-74, relay 36, strand 24, jack terminal 41. and cutofl' relay 63, to pole 10 of battery F. Current from battery F flowing -through this latter circuit will keep relay 36 energized and its armatures attracted, even if contact 42-46 is,opened, contacts 47-7 4 and l2-fl6 being in multiple .in the circuit of relay 36. Thus this armatures of relay 36 are locked in attracted position until the connecting plug is removed from the line jack C. The operator now rcceives from the party at substation A, in-

structions to connect his line for conversa- W tion with substation B, whereupon the operator takes the idle connecting plug of the pair and tests line B to ascertain whether it is busy or not, as will now be described. In making the busy-test on line B the opera- -tor merely touches the tip contact of the idle connecting plug E'against a terminal 5 of a jack C of line B, in the usual manner. If line B is, idle thejaclr terminals 5 5 will all be disconnected from the battery F at contact 67-61 which is then open, so that the testing plug will be unable to close any cuit from a jack terminal 5 of line B, which 1s connected to pole 10 of battery F as above described, through tip straml 11 of the testing plug 131, coutact-30-31 of relay 26, a portion (39 of the secondary of the operators induction coil, a portion of the winding of the operators receiver 70, impedance coil 75, and conductor 39, to pole J of battery F. V-Vhcu the operator touches the tip of the testing plug against the ring or sleeve 5 of a jack of line It, if line B is busy, an impulse of current from battery F flows through the last-dcscmbcd (.ll'CHlt and thereby produces a click in the oporatofls rcceivcr 70 which indicates to her that line B is busy, or in use. The loudness of the operators busy click can be regulated by regulating the resistance of the coil 75. It will be readily understood that plug E may be the answering; plug and plug I) the calling plug, if desired, in which case thebusytest will be made with plug D in a manner similar to that j ust' described in connection with plug E. Assuming that the line B is not in use, the operator inserts the idle connecting plug of the pair into a line jack C of line B, thereby closing a number of circuits through battery F which circuits will now be traced and explained as follows:

First. From pole 9 of battery F through conductor 39, the operatofis disconnect relay 26, strand 24 ot the calling plug, a jack- Sl')1'lllg-il: of line B, and cut-oil" relay 63, to pololO oi. battery F. Current from battery F flowing through this circuit energizes re lay 26-and thereby causes its armatures to be attracted and to open contacts EEO-31 and 32--33, and close contact 72-73. Now the operators telephone set and the busy-test arrangement are entirely out off from the talking strands 1.1 and 12 of the plug-cord circuit, at contacts 30-31 and 32-33 of relays 25 and 26. Contact 72-73 being closed closes a circuit from pole '10 of battery F, through conductor 38, supervisory lamp 40 which is associated with relay 26, contact 72-73 of relay 2G, and contact ill-42 of supervisory relay 43 which is associated with the calling plug, to pole 9 of battery F. Current from battery F flowing through this circuit lights the supervisory lamp 40 whiclnshows to the operator that the circuit oi line B is open. The current now flowing in circuit 39, 26, 24-, 4-, 63, also energizes the cutoff relay 63 of line B and 3 law-1 thereby causes its armature to be attracted and to open contacts 61-62 and 61- 66 and close contact 61-67. Contact 61-66 being open prevents line lamp 7 from displaying a signal when the circuit of the line is closed. Contact 61-62 being open disconnects pole 10 of battery F from limb 2 of line B. Contact 616T being closed connect's limb 2 of line B to the several line-jack terminals 5 5. 1

Second. From pole 9 of battery F through conductor 39, intermittent circuit-closer H, conductor 87, ringing relay 15 associated with the calling plug, contact 35-34 of locking relay 36, strand 24 of the calling plug, a jack-spring 4 of line B, and cut-ofli relay 63, to pole 10 of battery F. Current from battery F flowing through this circuit, when the circuit-closer H closes the circuit, energizes ranging relay 15 and thereby causes its armatures to be attracted and to close contacts 1620 and 18-21, respectively, armature 16 also opening contact 16-17 While the ringing relay 15 is energized, a circuit is closed from ringing genorator G, through conductor 22, contact 20-16 of ringing relay 15 which is associated with the calling plug, strand 11, gackspring 3, limb 1 of line B, the ringer and condenser at substation B, limb 2 of line B, contact 61-67 of relay 63, jack-terminal 5, strand 12 of the calling plug, contact 1821 of ringing relay 15, and conductor 23, back to the generator C. Current flowing from the generator G through this circuit rings.

the ringer at substation B and thereby signals the party at this telephone. The ringer at substation B is rung intermittently, due to the fact that the ringing generator circuit over line B is closed intermittently by ringing relay 15 which is energized intermittently from battery F through the interlnittent circuit-closer H.

'When the party at substation B answers his call and lifts his receiver from the,

switch-hook, he closes the circuit between limbs 1 and 2 of his line and thereby closes a circuit from pole 10 of battery F, through supervisory relay 43 which is associated with the calling plug, strand 12, jack-terminal 5 of line B, contact 67-61 of relay 63, line B, and line relay 60, to pole 9 of battery F. Current from battery F flowing through this circuit energizes relay 43 and thereby causes its armature to be attracted and to open contact ll--42 and close contact 4246. Contact 4].42 being open extinguishes lamp 40 which indicates to the operator that the party at substation B has answered his call. Contact 42-46 being closed closes a circuit from pole 9 of battery 1?, through contact 4246, locking relay 36, strand. 24 of the calling plug, jackspring 4 and relay 63, to pole 10 of battery F. Current from battery flowing through this circuit energizes relay 36 and thereby closed closes a circuit from pole 9 of battery F, through contact 47-74, locking relay 36, strand 24 of the calling plug, jack'spring 4 of line B, and relay 63, to pole 10 of battery F. Current from battery F flowing through this latter circuit will keep relay 36 energized and its armatures attracted, even if contact 42-46 is opened, contacts 47-74 and 4246 being in multiple in the circuit of relay 36. Thus the armatures of relay 36 are locked in attracted position until the connecting plug associated therewith is removed from the line jack C. Now the parties at substations A and B are connected for conversation with each other, over limbs 1 and 2 of their lines and talking strands 11 and 12 of the plug-cord circuit. When the conversation is completed the party at subline in a manner well understood and thereby stops the flow of current through the supervisory relay 43 which is associated with line A. This supervisory relay 43 is now deenergized whereupon the armature thereof releases and closes the back contact 42--41 which closes a circuit .from pole 9 of battery F, through contact 4241, contact 7372, supervisory lamp 40 which is associated with line A, and conductor 38, to

pole 10 of battery F. Current flowing from battery F through this latter circuit causes the supervisory lamp 40 to light and thereby station A hangs his receiver upon the switch-hook, which breaks the circuit of his convey to the operator the signal for dis connection of line The party at substation B by hanging his receiver upon the switch-hook, controls circuits and gives his disconnect signal to the operator the same as just described in con nection with line A. Now the operator, having received both disconnect signals from substations A and. B, withdraws the connecting plugs D and E from the line jacks and thereby opens the circuits of the locking relays 36 36 and the operators disconnect relays 25 26,jover the line connections, thus de'energizing allot the relays 36, 36, 25 and 26 and causing the armatures thereof to re lease. Relays 36 36 now close contacts 34-35 34 35 in the respective circuits of ringing'relays 15 15, but these latter relays are not energized by such contacts being closed because the circuits of the relays 1 5 15 lieover the line connections are now open at the connecting plugs. Relays- 25 and 26 now open the cont-acts 72-73 72-73 inthe respective supervisory lamp circuits and thereby extinguish the said lamps 40 40.

Now all of the circuits and apparatus are restored to normal. position and condition prepz'nratory 't'or another cycle of operations as ii t described.

It will be seen that the locking rclays T36 lays 3t; 36, while connection is still made with the lines, due to contacts =.l7--74l ii-74: being closed.

Attention called to my two co-pending, applications Serial No. 625,088, filed May 4, i till 1 and Serial No. (544,042, filed itugust let, 1

the appended claims.

ltliat l. rlauu as my invention is:

it. in a telephone system having lines, a

plug-cord connecting circuit tor-the lines, an operators telephone set including a re cciver, normally connected with the plugcord circuit, a relay associated with each of the connecting plugs of a pair for disco]? nccting the opcratofis telephone set from the respective connecting plugs when the litter" are connected with respective lines, and an i impedance coil connected from the operators telephone receiver to a source of electricity, whereby a busy-test circuit is normally established from a conductor oi either said connecting plug through contacts of the respective said relays, the operators tell phone receiver, and the said impedance coil, tor purposes substantially described.

A telephone system having lines, a plug-cord conncizting circuit tor the lines, a source ot' electricity, a plurality ot connection terminals for each of the lines, nfill (Hflllltilltut termina l. having a test contact, "the t st contacts for each of the lincsbcing connected together and. normally disconnected "from the said source of electricity, an operators telephone set normally connected with the plilg-cord connecting circuit,a relay tor disconnecting the operators telephone set from a connecting plug when the latter is Connected witha line, and an impedance coil connected from the operatofis telephone receiver to the said source of electricity, whereby a test circuit is normally established trom conductor ot a connecting cord and plug through a. contact ol the said relay, the operators telephone receiver, and the said im.pe dance coil, the said test circuit cooperating with the line test contacts for purposes sub stantially as described, and means for changn in,

ing the potential of the said line test cont acts when connection made with the line.

8. A telephone system having lines, a plug-cowl connecting circuit for the lines, a source of electricity, a plurality ol. conneclion terminals tor arch of the. lines, each connection terminal having a test contact, the test contacts for each of the lines being connected together and normally disconnect ed from the said source of electricity, an operatofls telephone set including a receiver, normally connected with the connecting plugs of the ung-cord connecting circuit. a

, pair ol. relays associated with the respective connecting plugs of a pair, said relays being adapted to discoimect the operations telephone set from the respective connecting plugs when the latter are connected with respcctive' lines, and an impedance coil connected from the ope 'ators telephone receiver to the source of electricity, whereby a test; circuit is normally established from a conductor of either connecting cord and plug of the pair through contacts of the respective said relays, the opcratofis telephone receiver, and the said impedance coil, said test circuit cooperating with the test contacts ol the lines for purposes substantially as described, and means for connecting the .aid lest contacts of the lines with the said source of electricity when connection is made with the lines.

l. in a telephone system having lines, a plug-cord connecting circuit for the lines, an operators telephone set normally connected with a comuwcting plug of the connecting circuit, a ringing relay associated with the said connecting plug and ar 'anged to auto-.

zuat-ic: lly send a ringing current over a line when connection is made therewith, an opcratofs disconnect relay associated with the said connecting plug and adapted to disconnect the opc'ators telephone set from the said plug when connection is made with a line, and an impedance coil connected from the operator's telephone receiver to a source of electricity, whereby a test circuit is normally established from a. conductor of the said connecting plug through a contact of the ringing relay, a contact of the ope 'ators disconnect relay, the operators telephone receiver, and the said impedance coil, to cooperate with llie lines for purposes substantially as described.

5. In a telephone system having lines, a plug-cord connecting circuit for the lines, the said connecting circuit tenninating in a pair of connecting plugs, an o .u-ratoi"s telephone set mirmally connected with the conuccting circuit, a ringing relay associated with each of the said connecting plugs for automatic: lly sending a ringing current over a line when connection is made between either of the said c nnecting plugs and a line, a pair of opcl-"ors disconnect relays associated with the said connecting plugs and adapted to disconnect the operators telephone set from the plug-cord connecting circuit when the said plugs are connected with respective lines, and an impedance 0011 connected from the operators telephone re ceiver to a source of electricity, whereby a test circuit is normally established from a conductor of either connecting plug of a pair through contacts of the respective ringing relays, contacts of the respective operators disconnect relays, the operators telephone receiver, and the said impedance coil, the said test circuit cooperating with the lines for purposes substantially as described.

6. A telephone system having lines, a plurality of connection terminals for each of the lines, a source of electricity, each of the said connection terminals having a test contact, the test contacts for each of the lines being connected together and normally disconnected from the said source of electricity, a plugcord connecting circuit for the lines, an operators telephone set normally connected with the plug-cord connecting circuit, a supervisory signal associated with each of the plugs of a pair, a pair of supervisory relays associated with each of the said supervisory signals for controlling the latter, one of the said supervisory relays of each pair being adapted to disconnect the operators telephone set from the corresponding con necting plugs when connection is made bctween the latter and respective lines,the

other said supervisory relay of each pair furnishing paths for connecting the test contacts of respective lines with the said source of electricity when connection is made between the corresponding connecting plugs and the lines, and an impedance coil connected from the operators telephone receiver to the source of electricity, whereby a test circuit is established from a conductor of each of the connecting plugs of a pair through contacts of the respective supervisory relays which control the operators telephone set, the operators telephone receiver, and the said impedance coil, the said test circuit cooperating with the said line test contacts for purposes substantially as described.

7. In a telephone system having lines, a plurality of connection terminals for each of the lines, a source of electricity, each of the said connection terminals having a test contact, the test contacts for each of the lines being connected together and normally disconnected from the said source of electricity,

a relay associated with each of the lines for connecting the said test contacts thereof with a limb of the line when connection is made with the line, a plug-cord connecting circuit for the lines, an operators telephone set nor- 1 mally connected with the plug-cord connecting circuit, a supervisory signal associated with a connecting plug of the plug-cord circuit, a pair of supervisory relays for con: trolling the said supervisory signal, one of the said supervisory ,relays being adapted to disconnect the operators telephone set from the corresponding connecting plug when connection is madebetween the latter and a line, the other said supervisory relay furnishing a path foi" connecting the test contacts of a line with the said source of electricity when the said connecting plug is connected with a line, and an impedance coil connected from the operators telephone receiver to the said source of electricity, where by a test circuit is established from a conductor of the said connecting plug through a contact of the said supervisory relay which controls the operators telephone set, the operators telephone receiver, and the said impedance coil, the said test circuit cooperating with the said line test contacts for purposes substantially as described.

8. In a telephone system having lines, a connecting circuit for the lines, said connecting circuit terminating in a pair of connecting plugs, an operators telephone receiver normally connected with a contact of each of the said connecting plugs, a relay associated with each of the said connecting plugs for disconnecting the said receiver from the said contacts of the respective connecting plugs 9. In, a telephone system having lines, a

plug-cord connecting circuit for the lines, an operators telephone set including a receiver, normally connected with the plugcord circuit, a relay associated with each of the connecting plugs of a pair for disconnecting the operators telephone set from the respective connecting plugs when the latter are connected with respective lines, whereby a busy-test circuit is normally established from a conductor of either said connecting plug through contacts of the respective relays and the operators telephone receiver, for purposes substantially as described.

10. In a telephone system having lines, a plug-cord connecting circuit for the lines, an operators telephone normally connected with the lug-cord circuit, and means for disconnecting the operators telephone from the respective connecting plugs when the latter are connected with lines, whereby a busy-test circuit is normally established from a conductor of either said connecting plugs through the operators telephone, for purposes substantially as described,

11.A telephone system having lines, a plug-cord connecting circuit for the lines, a source of electricity, a plurality of connection terminals for each of the lines, each connection terminal having a test contact, the test contacts for each of the lines being connected together and normally disconnected from the said source of electricity but adapted to be connected therewith, an 0perators telephone normally connected with the plug-cord connecting circuit, and a relay for disconnecting the operators telephone from a connecting plug when the latter is connected with a line, whereby a test circuit is normally established from a conductor of said connecting plug through a contact of the said relay and the operators telephone, the said test circuit cooperating with the line test contacts for purposes substantially as described.

12. A telephone system havin lines, a plug-cord connecting circuit for t e lines, a source of electricity, a plurality of connection terminals for each of the lines, each connection terminal having a test contact, the test contacts for each of the lines being connected together and normally discon-,

nected from the said source of electricity but adapted to be connected therewith, an op erators telephone normally connected with the plug-cord connecting c1rcuit,'and means for disconnecting the operators telephone from a connecting plug when the latter is connected with a line, whereby a test circuit is normally established from a conductor of said connecting plug through tl e operators telephone, the said'tcst circuitcooperating with the lineeest contacts for purposes substantially as described.

13. In a telephone system having lines,

said lines having suitable test contacts, a

plug-cord connecting circuit for the lines, an operators telephone, a supervisory signal, and a pair of relays for controlling the said supervisory signal, one of the said relays normally connecting the operators tele- ELMER R. CORWIN.

Witnesses:

Fmannnron R. PARKER, FRANK J. RYAN. 

